Hajjaj (may Allah have mercy on him) narrated to us; he said: Ibn Jurayj (may Allah have mercy on him) said: "Three quru'." And Ibn Jurayj (may Allah have mercy on him) narrates from Ata' al-Khurasani (may Allah have mercy on him), from Sayyiduna Ibn Abbas (may Allah be pleased with them both), that Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Three menstruations (that is, three quru' means three menstruations).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15398